Most gas analyzers require periodic calibration to ensure proper performance. A typical calibration process consists of feeding a calibration gas into the inlet of a gas analyzer. The concentration of calibration gas can range from parts per billion (ppb) to parts per million (ppm) and higher.
Calibration gas can be purchased compressed in large, heavy cylinders or tanks. Because most calibration gases are volatile, tanks of compressed calibration gas are inherently dangerous. For field use, smaller and often disposable cylinders are available, but these cylinders are expensive and the quantity of contained gas comparatively small. Cylinders of calibration gas also have a limited shelf life depending on the gas composition and concentration.
As an alternative to purchasing cylinders of compressed calibration gas, permeation devices can be used. A typical permeation device comprises a small container of a highly concentrated calibration gas, usually in liquid form. Part of the container consists of a permeable material that permits molecules of the gas to pass through at a controlled rate. This rate is extremely temperature dependent. Therefore, for a constant emission, the permeation device is usually maintained in a temperature controlled oven. A carrier gas can be moved past the permeation device to produce a blended concentration of the calibration gas. Such systems are not available for all calibration gases and the temperature control issue is problematic for field use. Even in the laboratory, stabilization of a permeation system can take a considerable amount of time and effort. Furthermore, permeation devices cannot be turned off, although refrigeration can slow the emission to a level low enough to assure a reasonable shelf life.
Electrochemical cells can also be used to produce a controlled quantity of calibration gas. Although electrochemical cells stabilize quickly and are available in compact packages, they cannot be used with ammonia and other important calibration gases, and replacement cells are costly.
In view of these and other deficiencies in the art, it would be highly desirable to provide new and improved apparatus and methods for generating calibration gas.
Accordingly, it is a purpose of the invention to provide new and improved apparatus for generating calibration gas that is easy to construct.
It is another purpose of the invention to provide new and improved apparatus for generating calibration gas that is easy to use.
It is still another purpose of the invention to provide new and improved apparatus for generating calibration gas that is inexpensive.
It is a further provision of the invention to provide new and improved apparatus for generating calibration gas that uses low-cost consumables.
It is still a further provision of the invention to provide new and improved apparatus for generating calibration gas that is not temperature dependent.
It is yet still a further purpose of the invention to provide new and improved apparatus for generating calibration gas that is lightweight and portable.
It is yet a further purpose of the invention to provide new and improved apparatus for generating calibration gas that is neither chemically nor physically hazardous.
It is another purpose of the invention to provide new and improved apparatus for generating calibration gas that provides a wide range of calibration gas concentrations.